We removed the compressors. You’re welcome.
How we replaced the compressors in our ultra-low freezers with the free-piston Stirling engine — and why that’s a huge benefit for you.
Unlike failure-prone cascade ULT systems, which use two compressors and cooling loops in a cascading arrangement, the simple free-piston Stirling engine design uses a reciprocating piston and displacer, along with a continuous, gravity-driven thermosiphon to cool the cabinet interior.
Compact system
located at the top
of the freezer
How does the free-piston
Stirling engine work?
- The advanced linear motor and thermosiphon system design is inherently more efficient and stable, with only two moving parts.
- The two moving parts, the piston and the displacer, expand and compress helium in the engine, absorbing heat from the thermosiphon, keeping the cabinet cold.
- The gravity-fed thermosiphon “hugs” the inner cabinet and is charged with 100 percent natural R170.
- Similar to steam condensing on the lid of a pot of boiling water, R170 refrigerant is condensed to liquid near the “expansion zone” of the Stirling engine and flows down the thermosiphon via gravity. As it absorbs heat from the interior cabinet of the freezer, it turns to gas, rising back up the tube where it is cooled back to a liquid and the process repeats.
How does the Stirling engine revolutionize ULT system reliability?
- The two moving parts never wear because gas bearings eliminate all physical contact (friction) during motion, like a puck on an air hockey table.
- There is no oil to clog up the system or lubrication levels to maintain.
- The system requires virtually no maintenance.
- Continuous modulation controls temperature like cruise control maintains speed in your vehicle. Compressor start/stop cycles would be like controlling your car’s speed by cycling the ignition on and off. These excess mechanical and electrical stress leads to failures and higher electrical usage.
- The thermosiphon is isothermal, driving superior interior temperature uniformity without high pressure or pumps.
The first and best ENERGY STAR®-certified ULT storage solution
At -80°C, Stirling Ultracold ULT freezers use 70–75 percent less energy than standard compressor-based ULT freezers. Our upright models meet the ENERGY STAR certification without using a performance-reducing “eco” mode that trades sample safety for energy rating.
Learn more about Stirling’s models here.
